The present invention relates to an improved method of bonding composite wood products using an adhesive mixture of a diisocyanate modified with an alkylene carbonate or triacetin. Significantly reduced press times and/or temperatures and improved product quality are achieved by the use of both thermal and radio frequency energy while pressing to cure the adhesive and produce the products
Composite wood products bonded with durable adhesives have been known for many years. These include well known materials such as particle board, flake board, and oriented strand board made from comminuted wood. They also include products such as plywood, laminated veneer lumber, laminated beams, and lumber reformed from thick veneers or sawn strips. All share the property that an intermediate form of wood; e.g., sawn boards, veneer or particles, are bonded with an adhesive, most typically a thermosetting adhesive, under heat and pressure to form a desired product configuration. Some more massive products, such as laminated beams, use room temperature setting adhesives since heat transfer into the interior is very slow and enormously long press cycles would otherwise be required.
In most plants making reconstituted wood products, the press is the most expensive piece of equipment. This is usually a massive hydraulic press that may have multiple openings for simultaneously pressing a plurality of panels. Alternatively, the press may be a continuous type in which the product is carried between moving steel belts. Presses are most usually heated internally with steam or hot oil. The products are held under heat and pressure until the heat has time to transfer to the center of the cross section and cure the bonding adhesive. In some instances the adhesive coated wood is preheated with microwave energy before entering the press. Unless very carefully controlled this can cause precure of the adhesive resulting in poor bonding. The press cycle acts as a bottleneck to limit production rate of the plant. Any way in which the cycle can be shortened can usually increase plant productivity with no or minimal additional capital investment.
The adhesives used for reconstituted wood products were for many years limited to heat cured thermosetting condensation resins; e.g., urea-formaldehyde, melamine-urea-formaldehyde, or phenol-formaldehyde types. Phenol-resorcinol-formaldehyde resins were and still are widely used for room temperature setting applications. With the exception of the urea based types, these resins give a water resistant bond that enable the products to be used in wet environments. More recently, isocyanate based adhesives have come into use. Most usually these are based on mixtures of diphenylmethane diisocyanate monomers and its oligomers, usually referred to as PMDI resins. The PMDI resins depend on ambient moisture and heat to cure to crosslinked polyureas. Products made from these resins also exhibit good water resistance.
While most reconstituted wood products use conductive thermal heat for curing the adhesives, some systems have used radio frequency (RF) energy in place of or to supplement conventional heating. Radio frequency energy has been commonly used for bonding finger joints in lumber products and for edge gluing narrower strips into wider pieces for furniture panels. It has also been recently used as a supplemental heat source in the manufacture of laminated veneer lumber, in redrying veneers having moisture content above specifications, and in the manufacture of laminated structural members.
Laminated veneer lumber has been in commercial production and use for a number of years, often as the tension member in trusses; e.g., as seen in Troutner, U.S. Pat. No. 3,813,842. It has the advantage that defects, particularly knots, do not run completely through the piece as they frequently do in solid sawn lumber. Laminated veneer lumber is normally laid up in panel form and ripped to the desired width after pressing. Bassett et al., in U.S. Pat. No. 6,001,452, describe lumber products made by selectively placing and adhesively bonding wood from the outer (and stronger) portion of the logs on the edge portions of their product.
PMDI adhesives are routinely used for manufacture of the flake boards now widely used in place of plywood for residential and commercial construction. These adhesives have not found as much use in bonding wood products such as laminated veneer lumber or plywood, however. Relatively high press temperatures must be used to bring cure times to economic levels. These temperatures frequently cause steam xe2x80x9cblowsxe2x80x9d or blisters in the product if the pressure is released rapidly. The gradual pressure release time thus required significantly extends the effective pressing cycle. Further, the high heat required can darken the wood and deleteriously affect appearance.
Gaul et al., in U.S. Pat. Nos. 4,344,798, 4,359,507, and 4,414,361, describe PMDI compositions modified with liquid esters of carbonic acid, particularly ethylene, propylene, or butylene carbonates, for bonding composite wood products. The modified compositions are said to reduce cost and viscosity of the resin so that it may be applied to wood particles using conventional equipment.
Hunter et al, in U.S. Pat. No. 5,140,086, show the use of propylene carbonate modified PMDI as an impregnant for cellulose sheets. Park et al., in U.S. Pat. No. 5,580,922, show a similar use of PMDI modified with up to about 20% triethyl citrate or triacetin.
An article by Vick in U. S. Forest Service, Southeastern Forest Research Experiment Station Research Paper SE-262 (1987) teaches the application of prelaminated veneer strips to the edges of construction lumber using emulsion-type urethane adhesives cured with the application of RF energy. However, the art appears to have ignored the possibility and advantages of using RF energy to cure composite wood products using PMDI or modified PMDI adhesives. One reason for this is believed to be the low polarity of PMDI resin molecules which results in a low acceptance of radio frequency energy. However, the present inventors have found that the use of PMDI resins modified to increase their polarity can be effectively cured by radio frequency energy. The present invention is directed to the use of these modified resins with radio frequency energy being applied during the pressing cycle of composite wood materials.
The invention is directed to the formation of composite wood products bonded with polyfunctional isocyanate adhesives that have been modified with compatible materials to increase their polarity and susceptibility to radio frequency energy. Modifiers should be mutually soluble in the isocyanate and water. The wood may be provided in the form of particles, flakes, strands or splinters, veneers, or sawn wood strips. The adhesive is preferably based on diphenylmethane diisocyanate which may be in monomeric form or mixed with oligomers of this and other materials and is generally referred to as PMDI. However, the adhesives may be selected from other lower aliphatic, alicyclic, or aromatic polyfunctional isocyanates. The modifiers are selected from the group of lower alkylene carbonates and/or triacetin. The preferred modifiers are ethylene, propylene or butylene carbonate. Of these propylene carbonate is the most preferred. From about 5-25% of the modifier is used, based on the weight of the isocyanate adhesive. A more preferred range is about 10-20% modifier.
In the practice of the method the wood particles are first at least partially coated with the above adhesive mixture. They are then formed into a preform such as a mat, multi-ply veneer layup, or other configuration suitable for application of heat and pressure to bond the assembly and form an intermediate or final product configuration. The preform is then subjected to predetermined pressure while both conductive thermal heat and radio frequency energy (RF) are applied. The thermal energy would typically be supplied from press platens heated to a given temperature with steam or oil. It is not essential that the radio frequency energy be applied through the entire pressing cycle. The term xe2x80x9cradio frequency heatingxe2x80x9d should be considered sufficiently broadly to indude both conventional dielectric heating frequencies in the 2 MHz to 200 MHz range and higher frequencies generally considered to be in the microwave range of about 900 MHz to 2400 MHz. Sufficient radio frequency energy is used so that the press cycle to achieve a handleable product is reduced to no more than about 80% of the time required when thermal energy alone is applied at the given platen temperature. Preferably the time is reduced to less than 70% and most preferably to less than 50% of the time required by the use of conductive heat alone at a given temperature.
By xe2x80x9chandleable productxe2x80x9d is meant one which may be subjected to normal post pressing handling, such as might occur during a plant production operation, without external or internal damage or debonding.
Using the modified adhesive with radio frequency energy during pressing, the press temperature required may be significantly reduced below that required for unmodified isocyanate. Temperatures in the range of 100-140xc2x0 C., preferably about 110-130xc2x0 C., are preferred. Press platen temperatures should be at least sufficient so that they will not act as a cold sink to withdraw heat from the preform during the application of radio frequency energy. They should also be sufficiently high to begin vaporization of any water contained within the wood particles since this water is essential to begin the polymerization of the isocyanate to polyurea compounds. The combination of the modified adhesives with radio frequency energy allows lower platen temperatures to be used. Among other advantages, this reduces surface scorching or discoloration of the pressed articles.
At a press platen temperature of 120xc2x0 C. a 25 ply parallel laminated veneer product about 38 mm (1xc2xd inches) thick has been cured in about 6 minutes by application of additional radio frequency energy at a level of about 32 kw/m2 (3 kw/ft2).
The ratio of radio frequency energy to conductive thermal energy will be variable and will depend on many factors. These include the specific equipment available and the configuration and nature of the products being formed. If the products are relatively thin, the ratio of radio frequency to conductive energy might be fairly low. For thicker products the ratio will preferably be considerably higher. At least about 10% of the energy supplied to the preform during pressing will be radio frequency energy. Most preferably the radio frequency energy will be in the range of 50-90% of the total energy input.
It is an object of the present invention to provide a method by which composite wood products may be rapidly bonded using isocyanate adhesives and a combination of conductive heat and radio frequency energy.
It is another object to provide modified isocyanate adhesives of increased polarity for use with the method, said adhesives being more acceptable of radio frequency energy than unmodified isocyanates.
It is a further object to provide a method in which the pressing times and temperatures for composite wood products are significantly reduced.